1. Field of the Invention
The present invention relates to a piezoelectric thin-film resonator and a method for producing the same.
2. Description of the Related Art
In piezoelectric thin-film resonators, a vibrating portion in which a piezoelectric thin-film is disposed between a pair of excitation electrodes facing each other must be acoustically separated from a substrate. For this purpose, the vibrating portion (membrane) must be partially lifted from the substrate with an air-gap layer therebetween.
In a piezoelectric thin-film resonator having this type of structure, a vibrating portion includes a supporting portion supported on a substrate and a lifting portion lifted from the substrate. The lifting portion is supported by the supporting portion, and thus a stress is easily concentrated in the vicinity of the boundary between the lifting portion and the supporting portion.
Japanese Examined Patent Application Publication No. 6-40611 discloses a piezoelectric thin-film resonator having such a structure. The piezoelectric thin-film resonator described here is often used as a terminal part for mobile communication or a part for BLUETOOTH™ communication devices. In recent years, devices themselves have been significantly reduced in size because the communication frequency has become higher. The frequency is determined by the following equation: Resonant frequency=frequency constant of the piezoelectric diaphragm/T, wherein T represents the thickness of the vibrating portion.
The frequency constant of the piezoelectric diaphragm is a constant determined by the piezoelectric material. For example, when a piezoelectric thin-film resonator is used in the 2 GHz band, the thickness of a piezoelectric film is about 2 μm, the thickness of each electrode is about 0.1 μm, and the thickness of an air-gap layer is about 1 μm.
As described above, the thickness of the air-gap layer has become very small and thus it is known that the finish of the electrodes provided on this air-gap layer having a small thickness significantly affects the characteristics of the piezoelectric thin-film resonator.
As described above, piezoelectric thin-film resonators have been significantly reduced in size as the communication frequency has become higher. Consequently, new problems, which have not been considered as important problems in the past, have arisen. The problems will be described below.
An air-gap layer is formed according to the following process. First, a sacrificial layer is formed on a substrate. A main portion of a piezoelectric thin-film resonator is formed on the surface of the sacrificial layer so as to be continuous with the surface of the substrate. Subsequently, the sacrificial layer is removed by etching or the like to form the air-gap layer.
When a crystalline material such as zinc oxide is used for the sacrificial layer, the smoothness of end surface portions of the sacrificial layer is generally deteriorated compared with the principal surface of the sacrificial layer. The surface roughness (Ra) of the end surface portions of the sacrificial layer is about 10 nm to about 20 nm. As described above, as the frequency becomes higher, the thickness of the sacrificial layer becomes very small. Consequently, the deterioration of the smoothness of the end surface portions causes the following problems:
1) The smoothness of a dielectric film provided on the sacrificial layer is also deteriorated. Furthermore, crystallinity of a lower electrode provided on the dielectric film is deteriorated, resulting in the increase in wiring resistance. Thereby, resonance characteristics are deteriorated.
2) It becomes difficult to cover the sacrificial layer with a thin dielectric film. On the other hand, when the thickness of the dielectric film increases in order to improve the covering ability, resonance characteristics are deteriorated.
3) If the covering by the dielectric film is insufficient, an area is formed where the sacrificial layer is in contact with the lower electrode. In such a case, when the sacrificial layer is removed, the lower electrode is corroded by an acid or the like used for removing the sacrificial layer. The corrosion of the electrode deteriorates resonance characteristics.
These problems become more obvious because as the communication frequency becomes higher, devices are reduced in size, and accordingly, the thickness of the air-gap layer becomes very small.